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The Ala

The ala was composed entirely of cavalry. The name is Latin for wing and derives from the use of horsemen on the flanks of an army, where they gave protection to the infantry centre. When necessary they could deliver flank attacks themselves and thereafter deploy against retreating enemy or attempt to divert pressure on a withdrawal of their own side. The large-scale employment of cavalry serving as auxilia was in large measure due to Caesar following his experience in Gaul. His Gallic units were originally led by their own chiefs and it seems probable that their internal organization was left to the commanders and local custom. Eventually the units became organized into troops (turmae). The alae were normally of a strength of roughly five hundred (quingenaria), but there were a few of the strength of roughly a thousand (millaria). By the second century the ala quingenaria were arranged into section of sixteen turmae, whereas the ala millaria were partitioned into twenty four turmae. So a turmae was not necessarily always of the same size.
Reports of a turmae in the ala quingenaria define the turmae at consisting of 32 men, and the total ala at 512 men.
The ala millaria is mentioned at the strength of 42 men in each turmae, making 1008 men for the the total strength of the ala.
The commander of the ala was a praefectus.

The ala was open to all. Normally only non-citizens were recruited, as the ala was understood to be a auxiliary force. But there was no bar on citizens entering who preferred the life of a horseman to that of a legionary.

At first the commander of an ala would have been a chief of his tribe taking his rightful place at the head of his troops. As the system became rationalized in the first century AD this command became a stepping stone in the career of young equestrians, i.e. men of the Roman knightly class.

The three main military commands (called the tres militiae) in this career were:
praefectus cohortis = commander of an auxiliary infantry
tribunus legionis = military tribune in a legion
praefectus alae = commander of an auxiliary cavalry unit

In the early empire, the men who aspired to these positions were mostly young equestrians or ex-centurions from the legions. The chief centurion of a legion, the primus pilus, automatically qualified for equestrian status and he could then obtain an independent military command in the auxilia, if he so wished.
But the rules were changed by the end of the first century AD and the position was then exclusively reserved for equestrians who had held at least some minor governmental position in the provinces of Rome.
This useful potential supply of educated provincials had become available as a result of the policy of Romanization and extention of the franchise deliberately pursued by the early emperors. It was a process developing in the auxilia itself. An uncouth barbarian from the frontier provinces or even beyond could join an auxiliary unit and soon became accustomed to the ways of civilized life. After completing his service he would retire as a Roman citizen and his sons could become legionaries and rise to the centurionate or stay in civil life and as citizens join a town council, etc...
One way or another the family could progress to the equestrian class and this in turn could be used as a stepping-stone by their children, who could rise to the higher ranks of army command or the civil service.
Hence auxiliary officers can be traced from almost all parts of the Roman world.

The troop commander was the decurio. A man promoted to this position could come from the lower ranks of the legion, since this was an accepted step from the ranks into the legionary centurionate.
The duties of an equestrian officer were:
"to keep the troops in camp, to bring them out for training, to keep the keys of the gates, to go round the guards from time to time, to attend soldiers' mealtimes and sample the food to prevent quartermasters from cheating, to punish offences, to hear complaints and inspect sick quarters."
It might almost be taken from a modern book of army regulations.